Q: App lock

You can only lock access to your phone altogether with a passcode/fingerprint, you cannot lock individual apps.

You might try Guided Access, which can lock the phone into a single app, and you set a separate code in order to unlock it: Use Guided Access with iPhone, iPad, and iPod touch - Apple Support. Be aware that someone can defeat Guided Access by force restarting the phone.

If you want the ability to lock apps, you can give Apple feedback at http://apple.com/feedback/iphone.html.